Interpretive Summary: Formulation of diets on a digestible amino acid basis allows for better predictions in growth performance and may decrease feed costs. The two most commonly accepted methods of determining amino acid digestibility use precision-fed adult roosters and ad libitum fed 3 week-old broiler chicks. The objective of this study was to compare amino acid digestibility coefficients of several feedstuffs using these two methods. This is one of the first studies to compare several different samples of the same ingredient among the two methods. Six corn, 8 co-products from the ethanol industry, and 3 animal by-products were obtained and evaluated and fed to both adult roosters and broilers and standardized amino acid digestibility coefficients were determined. For the corn samples, there were no differences in amino acid digestibility based on the method of determination. Any differences that did occur for amino acid digestibility in corn were not consistent among the amino acids and methods. For the ethanol co-products, there were some differences with the rooster assay yielding higher values, but these differences only occurred in 2 of the 4 samples. Animal by-products also yielded different amino acid digestibilities; the rooster assay yielding higher values for one sample and lower values for another. The results indicate that both these methods are acceptable for determining amino acid digestibility in feedstuffs; however, these procedures did not always yield similar results for all feed samples evaluated.

Technical Abstract:
The objective of this study was to evaluate and compare amino acid digestibility of several feedstuffs using 2 commonly accepted methods: the precision-fed cecectomized rooster assay (PFR) and the standardized ileal amino acid assay (SIAAD). Six corn, 6 corn distillers dried grains with or without solubles (DDGS/DDG), one wet distillers grains, one condensed solubles, 2 meat and bone meal (MBM) and a poultry byproduct meal were evaluated. Due to insufficient amounts, the wet distillers grains and condensed solubles were only evaluated in roosters. Standardized amino acid digestibility varied among the feed ingredients and among samples of the same ingredient for both methods. For corn, there were generally no differences in amino acid digestibility between the 2 methods. When differences did occur, there was no consistent pattern among the individual amino acids and methods. Standardized amino acid digestibility was not different between the 2 methods for 4 of the DDG samples; however, the PFR yielded higher digestibility values for a high protein DDG and a conventionally processed DDGS. The PFR yielded higher amino acid digestibility values than the SIAAD for several amino acids in 1 MBM and the poultry byproduct meal, but it yielded lower digestibility values for the other MBM. Overall, there were no consistent differences between methods for amino acid digestibility values. In conclusion, the PFR and SIAAD methods are acceptable for determining amino acid digestibility. However, these procedures do not always yield similar results for all feedstuffs evaluated. Thus, further studies are needed to understand the underlying causes in this variability.